Numerous epidemiological studies have established that there is a disproportionately high frequency of adverse cardiovascular events around 9 AM (e.g., sudden cardiac death, myocardial infarction). The mechanisms have been loosely attributed to behavioral stressors occurring at specific times of day (e.g., change in posture or activity). But, the independent role of the internal body clock (circadian pacemaker) in this day/night pattern has never been studied with appropriate techniques. Our preliminary data show a marked endogenous circadian rhythm in some markers of cardiovascular risk that may be implicated in the day/night pattern of adverse cardiovascular events. Thus, we have two Specific Aims: (1) Determine the effect upon cardiovascular risk markers of the circadian pacemaker (independent from behaviors); (2) Determine the interaction between circadian and behavioral influences upon cardiovascular risk markers (i.e., an interaction by which behavioral stressors have different effects at different circadian phases). To achieve these aims, we will utilize an intensive 12-day "Forced Desynchrony" protocol, wherein subjects' standardized behaviors and sleep occur at all phases of the circadian cycle. As this will be the first formal study in this area, we will begin by studying healthy individuals to define the normal human physiological rhythms and responses that could be implicated in the day/night pattern of cardiovascular risk. During the protocol standardized behavioral stressors include changes in posture (tilt table), activity (bicycle exercise) and sleep/wake state. The measures that can be implicated in triggering adverse cardiovascular events in vulnerable individuals, will include (i) autonomic nervous system activity (catecholamines, sympatho-vagal balance); (ii) hemodynamic factors (heart rate, blood pressure); (iii) Hemostatic factors (platelet aggregability and count); and (iv) vascular endothelial function (flow mediated vasodilatation). Based on preliminary data, we anticipate that behavioral stressors will have different effects at specific phases of the circadian cycle revealing vulnerable times for certain behaviors (likely corresponding to the peak in cardiovascular vulnerability from epidemiological studies). This project may ultimately help us to rationalize chronobiological therapies for cardiovascular diseases in terms of modification of behavior and timed pharmacological intervention. [unreadable] [unreadable]